Add cipher abstraction and test cases for OFB block mode

Adds OFB as additional block mode in the cipher abstraction, and additional
test cases for that block mode.
diff --git a/ChangeLog b/ChangeLog
index 4fbdb3d..80574f7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,6 +5,8 @@
 Features
    * Add support for ARIA cipher (RFC 5794) and associated TLS ciphersuites
      (RFC 6209). Disabled by default, see MBEDTLS_ARIA_C in config.h
+   * Add additional block mode, OFB (Output Feedback), to the AES module and
+     cipher abstraction module.
 
 API Changes
    * Extend the platform module with a util component that contains